1. Field of the Invention
The present invention relates to methods and apparatus for producing electrical energy. Particularly, the present invention is directed to an electrochemical fuel cell that produces electricity by irradiating an electrode of the fuel cell with a light source.
2. Description of Related Art
A variety of fuel cell devices are known in the art for generating electric power. Of such devices, most include graphite anodes and cathodes comprising a finely dispersed platinum catalyst.
For example, a phosphoric acid fuel cell (PAFC) is a power generation cell which employs a porous electrolyte layer of silicon carbide matrix for retaining concentrated phosphoric acid. The electrolyte layer is interposed between carbon-based electrodes (an anode and a cathode) to form an electrolyte electrode assembly, sometimes referred to as a membrane electrode assembly (“MEA”). The membrane electrode assembly is then interposed between electrically conductive bipolar plates. The electrolyte electrode assembly and the bipolar plates form a single fuel cell for generating electricity by reacting a fuel such as hydrogen with oxygen across the electrolyte. A single fuel cell as described generally herein has an electrical power output of about 0.8 volts. To raise the voltage of the electrical output, a fuel cell stack can be formed by arranging any desired number of fuel cells in electrical series on top of one another. Since the bipolar plates are electrically conductive, current flows through the stack via the end plates.
Another type of fuel cell device is a solid polymer electrolyte fuel cell which employs a membrane electrode assembly including electrodes separated by a polymer ion exchange membrane (proton exchange membrane or PEM). Similarly, the membrane electrode assembly and the bipolar plates make up a unit of the power generation cell. Once again, a predetermined number of the power generation cells can be stacked together to form a fuel cell stack having a desired output voltage.
In the fuel cell stacks, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The anode includes a catalyst that induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current.
The fuel cell should be operated at or near an optimum temperature for the performance of power generation. Generally, fuel cells known in the art operate at temperatures significantly above ambient or room temperature (e.g., 75° F.). The optimum temperature for operation can vary with each type of fuel cell system. For example, a phosphoric acid fuel cell is operated in the temperature range of 120° C. to 200° C., and a solid polymer electrolyte fuel cell is operated in the temperature range of 60° C. to 90° C. In order to maintain the temperature of the power generation cells in the desirable temperature range, various cooling systems have been adopted. Typically, the power generation cells are cooled by supplying coolant such as water to a coolant passage formed in the bipolar plates of the fuel cell stack.
Generally, fuel cells provide an environmentally clean alternative to energy production from fossil fuel combustion. The electrochemical efficiency of a fuel cell (currently ≈65%) handily exceeds that of internal combustion engines (<30%). However, in spite of recent increases in the prices of crude oil and natural gas, fossil fuel combustion continues to hold a significant economic advantage over fuel cells. The high cost of fuel cell energy production is attributable to the need for addition of expensive catalysts (platinum) to accelerate the oxidation of the fuel (hydrogen) at the anode and the reduction of oxygen at the cathode. The slow oxygen reduction reaction alone accounts for the largest limitation to the fuel cell efficiency, even in the presence of platinum catalyst.
The demand by dioxygen molecules (O2) for electrons during reduction at electropositive metal electrodes forces the electrostatic potential of the metal to wander negatively (referred to as a large negative overvoltage) before giving up the electrons needed by oxygen. Because the output power of a fuel cell is defined by the product of the cell potential (V) and current (I), i.e., P=VI, a drop in the potential lowers the power output and the cell efficiency linearly. Typically, a platinum-catalyzed hydrogen fuel cell operates at moderate current levels at cell voltages close to +0.75 volts instead of the thermodynamic equilibrium cell voltage of +1.23 volts. This amounts to an overvoltage of −0.48 volts below the thermodynamic voltage, thus limiting the efficiency to near 60%. Experiments probing alternatives to platinum catalysis of the electroreduction of oxygen continue to define a vigorous area of research. Nonetheless, platinum remains the best known electrocatalyst of the oxygen reduction reaction. In recent years, advances in fuel cell utility have relied upon improved methods for dispersing the platinum catalyst only at the active sites of graphite electrodes. The total amount of platinum needed to sustain operational currents was thereby reduced, along with the overall cost of fuel cells. However, the fundamental limitation caused by the overvoltage problem still persists.
Thus, there remains a compelling need in the art for a fuel cell that is more electrochemically efficient and/or more cost effective than known fuel cells. There is also a continuing need for a fuel cell system that can be operated at lower temperatures, such as at room temperature. The present invention provides a solution for these problems.